Christchurch Earthquake 2011
OPINION: What is acceptable risk and what are tolerable impacts of future natural hazard events like earthquakes and flooding? DR KELVIN BERRYMAN of GNS Science examines these questions.
As a scientist I am charged with bringing evidence forward so that policy-makers and the public can make informed decisions around risk of all sorts.
The concept of "no risk" is unattainable. Instead, we must identify the weak links in our built environment and weaknesses in community resilience that need urgent attention.
Since September 4, 2010, the greater Christchurch area has been subjected to natural hazard impacts not seen in New Zealand for 80 years.
Earthquakes and recent flooding have posed complex questions of our society and economy in terms of resilience.
For the first time in many decades our collective attention is on what we expect from the built environment around us.
How safe are we and what are reasonable expectations?
Following the Canterbury earthquakes of 2010-11, the royal commission spent considerable time discussing earthquake code provisions and the public have been vocal in the call for "safe buildings".
Without meaningful dialogue of what constitutes a "safe building", reactive provisions have been added to the earthquake prone building policy now before Parliament.
The Canterbury earthquakes - particularly the February 22, 2011, event - were far stronger than accounted for in earthquake provisions of the building code.
The level of ground shaking was equivalent to that required for critical facilities (that is, hospitals and other emergency service facilities) and about twice as strong as code requirements for normal buildings. Given these extreme levels, was performance acceptable?
Clearly when lives are lost, the obvious reaction is - no it was not.
The lives lost in Christchurch were a major tragedy but we need to also remember that in 2013 there were 254 road deaths in New Zealand and that, on average, the chances of being killed on New Zealand roads is about 100 times more likely than being killed in an earthquake.
The royal commission identified particular issues with two multi-storey office buildings and the weakness of elements of unreinforced masonry buildings, such as poorly-secured parapets.
However, it must be said that many older buildings constructed pre-1976 that would have been expected to collapse under such conditions did not do so, bringing into question just how weak older buildings are.
A similar finding emerged when 1950s era state houses in Lower Hutt were tested for simulated earthquake destruction and were found to be far more resilient than expected. It will be an extraordinarily conservative and costly step to demand retrofit of all buildings just because they are now 40 or more years old.
Risk to life depends on occupancy and the probability of the hazardous event, not just the structural characteristics of the building. In terms of economic risk it is likely infrastructure - roads, railways, ports, underground water and wastewater, and stop-banks - is weaker than we would expect and potentially represents major economic liabilities.
If we can calculate possible life risk threats from natural hazard events and then compare them with other risks we face in everyday life, can we identify the level of economic impact that towns, regions and the nation could withstand that could be said to be tolerable?
The science community is increasingly suggesting a useful initial indicator of "tolerable losses" could be a percentage of the regional economy that could be temporarily lost as a basis of considering "a tolerable limit" to economic losses.
If regions were to accept this or similar numbers, then mitigation of some sort would become an imperative if the threat presents as a larger potential loss.
It is difficult to make a financial case to mitigate rare events unless the consequences would be catastrophic. This makes the Canterbury earthquakes a truly vexing case - their impacts were devastating but they were very rare in their recurrence and severity.
In the floods in Christchurch earlier this month, the rainfall was estimated to be of the order of a 1-in-100-year return period event but with considerable variation across the city. With this sort of return period it is close to failing the council's requirement under the Building Act 1991 to ensure that floor levels are constructed so that habitable buildings do not become inundated in a 1-in-50-year storm event.
Clearly this is appreciated by the city council and Cera, and forecasts that a problem exists as a result of changed land levels, and reduced flood capacity in the rivers and the estuary following the earthquake sequence, may have played out.
The council is working on several mitigation works at present but cost-effective mitigation measures for all of the acknowledged increased flood risk in Christchurch are clearly a significant policy issue for Christchurch and its elected representatives and officials.
Moving forward, the solution is in partnerships between science, policy and the public.
Science brings evidence, the community provides expectations (calibrated by knowledge of benefit-cost of mitigation and the value of enhanced social resilience), and policy finds the "sweet spot" using evidence and realistic community expectation.
Our sponsorship of next week's third annual Seismics and the City forum gives us an opportunity to share important scientific knowledge and build those partnerships alongside other key players in the rebuild of greater Christchurch.
Dr Kelvin Berryman is director of the Natural Hazards Research Platform, GNS Science. He will be a speaker at Seismics and the City 2014 - Building Momentum.
Seismics and the City 2014 - Building Momentum will be held on Friday, March 28, at Rydges Latimer, Christchurch. More information: smartnet.co.nz.
- The Press
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